1. Field of the Invention
The present invention relates to an exposure apparatus. More particularly, the present invention relates to an exposure apparatus that allows light beams emitted from an organic light-emitting diode (OLED) to be focused such that images by the light beams overlap each other on the surface of a photosensitive medium.
2. Description of the Related Art
An exposure apparatus is an apparatus that scans a light beam onto a photosensitive medium to form an electrostatic latent image that corresponds to an image desired to be printed in an electrophotographic image forming apparatus.
The exposure apparatus is classified into a laser-scanning type exposure apparatus and a printer-head type exposure apparatus according to a light beam scanning type and how a light beam is scanned.
The laser-scanning type exposure apparatus is an exposure apparatus in which a light beam emitted from one light source forms an electrostatic latent image while passing along a main scanning direction of a photosensitive medium.
Alternatively, the printer-head type exposure apparatus is an apparatus in which an array of light-emitting sources (LEDs or OLEDs) is arranged in the lengthwise direction of a photosensitive medium and light beams emitted from the light-emitting sources are illuminated onto the photosensitive medium to form an electrostatic latent image.
One example of the printer-head type exposure apparatus is disclosed in U.S. Pat. No. 6,816,181 B2.
FIG. 1 is an exemplary perspective view of a printer-head type exposure apparatus of the related art. FIG. 2 is an elevational view in cross section of the exposure apparatus taken along a line A-A′ illustrated in FIG. 1.
Referring to FIGS. 1 and 2, the exposure apparatus 10 includes a printer head 19, a driving device 52 applying a voltage, and a controller 50 outputting a control signal that controls the driving device 52.
The printer head 19 has an organic electroluminescence (EL) element array, a transparent substrate 12, and a convex microlens array 16.
The organic EL element array is disposed on the upper side of the transparent substrate 12 and each organic EL element is spaced with a predetermined interval in the lengthwise direction of the transparent substrate 12. The organic EL element array includes a plurality of transparent electrodes 20 (used as an anode) disposed perpendicularly with respect to the length direction of the transparent substrate 12. An organic compound layer 22 is stacked on the transparent electrodes 20 in the lengthwise direction of the transparent substrate 12. A metal electrode 24 (used as a cathode) is disposed on the organic compound layer 22. The portion 18 where the organic compound layer 22, the transparent electrode 20, and the metal electrode 24 overlap one another serves as a light-emitting source that emits light.
The transparent electrode 20 has light transmissivity of more than 50% within a wavelength region of 400-700 nm of a visual light. The organic compound layer 22 may be a light-emitting layer consisting of one layer, or may include other layers (for example, a hole injection layer, a hole transfer layer, an electron injection layer, and an electron transfer layer) besides a light-emitting layer.
The transparent substrate 12 is disposed between the organic EL element array and the convex microlens array 16 to transmit light emitted from the light-emitting source 18 through the convex microlens array 16.
The convex microlens arrays 16 are disposed on the lower side of the transparent substrate 12 along the lengthwise direction. The plurality of convex microlens arrays 16 that correspond to the plurality of light-emitting sources 18 are disposed not to overlap each other. The convex microlens array 16 focuses light so that light that has passed through the transparent substrate 12 may be focused onto the photosensitive body 14 as an exposure spot 28.
In the printer head 19, a space that has a size the same as or greater than that of the light-emitting source 18 is formed between two adjacent light-emitting sources 18 in the main-scanning direction of the printer head 19. Therefore, a space that has a size the same as or greater than that of one exposure spot 28 is formed between exposure spots formed by light emitted from the adjacent light-emitting sources and focused on the photosensitive body 14 by the convex microlens array 16.
Therefore, to fill the space between the exposure spots 28 in the main-scanning direction, the printer head 19 should move as much as the vacant space between two adjacent light-emitting sources 18 in the main-scanning direction according to the inputted image signal.
Accordingly, the function of moving the printer head 19 in the main-scanning direction to fill the vacant space between the two adjacent exposure spots 28 should be added to the controller 50 and the driving device 52 in addition to the function of turning on or off the respective light-emitting sources 18. Thus, because mechanical elements are added, a driving structure itself is complicated. Also, when a microlens has one-to-one correspondence to each light-emitting source 18, a focal length is difficult to control and the depth of focus is short, so that a control error is sensitive when an apparatus is mounted.
Accordingly, a need exists for an improved exposure apparatus in which images scanned onto a photosensitive body overlap one another.